1. Field of Invention
The invention relates to a shift control system of an automatic transmission for a vehicle, and more particularly, to a shift control system that updates a relationship between an engaging torque of a friction element that is engaged to establish a shift stage and a command value that controls the engaging torque so as to prevent a shift.
2. Description of Related Art
In a vehicle provided with an automatic transmission that allows establishment of a predetermined gear stage by engaging a friction element, a command value is transmitted to a regulator that regulates an engaging torque of the friction element, that is, engaging capacity, or a solenoid valve that drives the regulator such that the required engaging torque of the friction element is generated during a shifting operation. In this case, based on a preliminarily stored relationship between a predetermined engaging torque of the friction element and a command value that allows generation of the engaging torque, the command value is determined and output in accordance with an actually required engaging torque.
For example, Japanese Patent Application Laid-Open No. JP-A-10-2273554 discloses an automatic transmission where a predetermined friction element selected among a plurality of friction elements is engaged or disengaged by controlling a working hydraulic pressure of a hydraulic actuator that operates in response to a command value transmitted from a control unit so as to perform shifting operations, and the command value is derived from a relationship (characteristics) of the working hydraulic pressure with a predetermined command value as being inherent to the hydraulic actuator. In the publication, a required torque capacity of the friction element at starting the inertia phase, that is, the required working hydraulic pressure in accordance with the required engaging torque is determined. This makes it possible to correct the aforementioned relationship such that the required working hydraulic pressure at starting the inertia phase may be correlated with the command value to the hydraulic actuator at starting the inertia phase with each other.
The aforementioned shift control device for an automatic transmission for a vehicle determines the required working hydraulic pressure in correspondence with the required engaging torque at starting the inertia phase during shifting. The relationship between the required working hydraulic pressure at starting the inertia phase and the command value output to the hydraulic actuator for generating the required working hydraulic pressure is then corrected such that those values are correlated with each other. As the engine speed or the rotating speed of the input shaft that is obtained in the condition which is susceptible to the outer disturbance is used to calculate the required engaging torque, quality or accuracy of learning of the aforementioned relationship cannot be sufficiently ensured. This may fail to sufficiently prevent a shift shock.